1. Field of the Invention
The present invention relates to an electron emission element and an image output device. The electron emission element of the present invention is applicable to, for example, an electron beam source for a thin display or an emitter portion of a vacuum device.
2. Description of the Related Art
In recent years, as an electron beam source for a flat display, and as an emitter portion of a vacuum device that can be operated at high speed, a cold cathode electron source has been considered. There are various types of cold cathode electron sources. In particular, a field emission (FE)-type, a tunnel injection (MIM, MIS)-type, a surface conduction (SC)-type, and the like are known.
In an FE-type electron source, an electric field is applied to a cone-shaped projection (electron emission portion) made of silicon (Si), molybdenum (Mo), or the like, whereby electrons are emitted from the top of the projection. In an SC-type electron source, an electric current is allowed to flow in an in-plane direction of a thin film formed on a substrate, whereby electrons are partially taken out of a previously formed crack portion in the thin film. Furthermore, in MIM-type and MIS-type electron sources, a layered structure (e.g., metal/insulator/metal (or semiconductor)) is formed, and electrons are injected through the metal side, and the injected electrons are partially taken out of an electron emission portion. The MIM-type and MIS-type electron sources have problems in that an element is likely to generate heat, electron emission characteristics become unstable, and element life becomes short.
The above-mentioned elements are advantageous in that they can be minimized and integrated by using fine processing technology. These elements also are advantageous in that heating is not required, unlike a hot cathode electron source.
FIG. 5 shows an example of an FE-type electron emission element. Referring to FIG. 5, a conventional electron emission element 1 includes a substrate 2, a cathode 3 formed on the substrate 2, a cone-shaped electron emission member 4 disposed on the cathode 3, an anode 5 opposed to the cathode 3, a control electrode 6 disposed between the cathode 3 and the anode 5, and an insulating layer 7 supporting the control electrode 6. Furthermore, FIG. 6 schematically shows an equipotential surface 8 between the cathode 3 and the anode 5 and a path 9 of emitted electrons during operation.
In the FE-type electron emission element, a high electric field is applied between the electron emission member and the control electrode, whereby electrons are emitted. The emitted electrons are accelerated by an electric field (lower than that between the electron emission member and the control electrode) between the anode and the control electrode, thereby reaching the surface of the anode. When the above-mentioned electron emission element is used for a flat display, a phosphor film is formed on the surface of the anode, and the emitted electrons cause the phosphor film to emit light.
In this case, in order to allow the electron emission member made of Si, Mo, or the like to emit electrons, a very high electric field is required. Accordingly, in the conventional electron emission element, the electric field intensity between the electron emission member and the control electrode is prescribed to be much larger than that between the anode and the control electrode.
A high electric field intensity between the electron emission member and the control electrode is obtained by applying a voltage therebetween. On the other hand, in order to prevent breakdown between the electrodes, it is very difficult to apply a high voltage between the electron emission member and the control electrode. Therefore, it is required to control the distance between the electron emission member and the control electrode in a minute region so as to obtain a high electric field at a low voltage. Therefore, there is a problem that the conventional electron emission element is difficult to produce.
Furthermore, as shown in FIG. 6, in the conventional electron emission element 1, the equipotential surface 8 is convex toward the anode 5. Therefore, while moving toward the anode 5, a group of emitted electrons have their path 9 bent and are diffused. However, diffusion of electron beams is not preferable for a flat display with minute pixels. Because of this, when the conventional electron emission element 1 is applied to a display, it is required to add a focusing electrode for preventing diffusion of electron beams. The addition of such a focusing electrode complicates the structure of a display, which leads to a decrease in production yield and an increase in production cost.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an electron emission element that is capable of emitting highly focused electrons and is produced easily, and an image output device using the same.
In order to achieve the above-mentioned object, the first electron emission element of the present invention includes: a cathode; an anode opposed to the cathode; an electron emission member disposed on the cathode; and a control electrode disposed between the cathode and the anode, wherein, during operation, the electric field intensity immediately above the electron emission member is lower than that between the control electrode and the anode. In the present specification, xe2x80x9cimmediately above the electron emission memberxe2x80x9d refers to a space above the electron emission member and below the control electrode. Furthermore, in order to achieve the above-mentioned object, the second electron emission element of the present invention includes: a cathode; an anode opposed to the cathode; an electron emission member disposed on the cathode; and a control electrode disposed between the cathode and the anode, wherein, during operation, a spatial average of the electric field intensity between the electron emission member and the control electrode is lower than that between the control electrode and the anode. In the first and second electron emission elements, electrons are emitted due to an electric field formed by a voltage applied between the electron emission member and the anode, and the amount of electrons to be emitted is controlled by the control electrode disposed between the electron emission member and the anode. Therefore, in the first and second electron emission elements, the distance between the electron emission member and the control electrode is not required to be minute. Accordingly, the electron emission element of the present invention is produced easily.
In the first and second electron emission elements, a voltage required for the electron emission member to emit electrons is applied between the electron emission member and the anode, and emission of electrons from the electron emission member is controlled by changing an electric potential of the control electrode. According to this structure, the emission of electrons from the electron emission member can be controlled easily. Herein, the voltage required for the electron emission member to emit electrons refers to the value of a voltage at which electrons are emitted from the electron emission member irrespective of the presence or absence of the control electrode. Furthermore, it is preferable that, while the emission amount of electrons is controlled by the control electrode (i.e., during operation of the device), a spatial average of the electric field intensity between the electron emission member and the control electrode is xc2xd or less (more preferably ⅓) of that between the anode and the control electrode.
In the first and second electron emission elements, it is preferable that, during emission of electrons, an equipotential surface in a space immediately above the electron emission member has a curvature that is convex toward the electron emission member. According to this structure, an electron emission element can be obtained that emits highly focused electrons.
In the first and second electron emission elements, it is preferable that the electron emission member is a thin film. According to this structure, a projection at which an electric field is concentrated is not used, so that an equipotential surface with a curvature that is convex toward the electron emission member is formed easily.
In the first and second electron emission elements, it is preferable that the electron emission member contains an allotrope of carbon (C). According to this structure, an electron emission element with a high electron emission ability can be obtained.
In the first and second electron emission elements, it is preferable that the allotrope includes diamond. According to this structure, an electron emission element with a particularly high electron emission ability can be obtained. In this case, when the diamond has a region that is terminated with hydrogen on its surface, an electron emission element can be obtained that is stable and has a much higher electron emission ability.
In the first and second electron emission elements, it is preferable that the allotrope includes an allotrope of carbon having a graphene structure. According to this structure, an electron emission element with a particularly high electron emission ability can be obtained.
In the first and second electron emission elements, it is preferable that the allotrope includes a carbon nanotube. According to this structure, an electron emission element with a particularly high electron emission ability can be obtained.
The first image output device of the present invention includes a substantially vacuum container and a plurality of electron emission elements disposed in a matrix in the container, wherein the electron emission element is the above-mentioned first electron emission element, and the device further includes a phosphor film disposed between the electron emission members and the anode. The second image output device of the present invention includes a substantially vacuum container and a plurality of electron emission elements disposed in a matrix in the container, wherein the electron emission element is the above-mentioned second electron emission element, and the device further includes a phosphor film disposed between the electron emission members and the anode. Since the first and second image output devices include the electron emission element of the present invention, they can be produced easily, and enable a high-resolution image to be displayed.
In the first and second image output devices, it is preferable that a voltage required for the electron emission member to emit electrons is applied between the electron emission member and the anode, and emission of electrons from the electron emission member is controlled by changing an electric potential of the control electrode. Furthermore, it is preferable that, while the emission amount of electrons is controlled by the control electrode (i.e., during operation of the device), a spatial average of the electric field intensity between the electron emission member and the control electrode is xc2xd or less (more preferably ⅓) of that between the anode and the control electrode.
In the first and second image output devices, it is preferable that, during emission of electrons, an equipotential surface in a space immediately above the electron emission member has a curvature that is convex toward the electron emission member.
In the first and second image output devices, it is preferable that the electron emission member is a thin film.
In the first and second image output devices, it is preferable that the electron emission member contains an allotrope of carbon (C).
In the first and second image output devices, it is preferable that the allotrope includes diamond.
In the first and second image output devices, it is preferable that the allotrope includes an allotrope of carbon having a graphene structure.
In the first and second image output devices, it is preferable that the allotrope includes a carbon nanotube.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.